The so-called "simulcast" methodology currently favored by the Federal Communications Commission as the basis for advanced television broadcasting in the United States, involves transmitting a conventional television signal, for example NTSC, over a first television channel, and transmitting a high definition television (HDTV) signal over an additional channel which would be assigned to each broadcaster. Since the assignment of an additional channel to each broadcaster will involve the use of those channels currently designated as "taboo" channels (i.e. those restricted for use in a given location), use of those channels will require that a Way be found to prevent or minimize within acceptable limits, the interference caused by or to these additional broadcast signals with respect to the existing conventional signals.
The copending parent applications describe methods and apparatus for implementing a "simulcast" television signal which will eliminate, or minimize to acceptable levels, interference to and from a conventional television co-channel signal.
Although reference will be made herein to NTSC signals for the sake of brevity and example, it is to be understood that the invention would apply as well to any conventional composite television system (i.e. PAL, SECAM).
The systems described in the parent applications seek among other things, to minimize interference between a high definition television (HDTV) signal and an NTSC co-channel signal, by utilizing a plurality of subcarriers which are placed within the HDTV signal frequency spectrum in a manner which avoids, or minimizes interference with the picture, chroma and sound subcarriers of the NTSC co-channel signal. As described in the '383 application, these subcarriers are preferably modulated with digital information, utilizing, for example, quadrature amplitude modulation. The '383 and '006 applications further describe grouping digitally encoded television information according to relative perceptual importance, and modulating more important ("higher priority") data and relatively less important ("lower priority") data on different carriers in a manner which makes the higher priority data more robust and less susceptable to interference from an NTSC signal provided on a co-channel (i.e. "taboo" channel) or the susceptibility of a signal component to drop out due to reduced signal strength.
The parent applications describe system which suitably shape the spectrum of the coded signal in order to provide different signal-to-noise ratios for a number of carriers which are then frequency multiplexed to form the "simulcast" television signal. This provides the flexibility of conveying different types of information (e.g. different codewords or different bits) via different carriers. Each of the carriers can be positioned within the frequency spectrum of the "simulcast" television signal so that they minimize interference with the picture and color subcarriers of a co channel NTSC signal.
If received signal-to-noise power ratio (SNR) is above a certain minimum SNR, then virtually error-free transmission/reception is possible. SNR is dependent on the choice of the modulation strategy employed for digital transmission. Thus the coverage area can be increased by a combination of increasing the transmitter power and/or resorting to complex digital transmission techniques, trading complexity with reduced SNR requirements.
"Multicarrier Modulation" (MCM) is a technique in which serial data is transmitted by separating it into several parallel data streams which are then used to modulate a large number of orthogonal carriers. This technique has been referred to as orthogonally modulated QAM, orthogonal frequency division multiplexing (OFDM) or dynamically assigned multiple QAM in the references incorporated herein. The technique will be referred herein as MCM to refer to any or all of these methods of multicarrier modulation, or variations thereof.
The basic MCM principle consists of splitting a high digital bit rate (representing the signal to be transmitted) into numerous carriers carrying a low bit rate. In order to achieve a high spectral efficiency, the spectra of the carriers are overlapped. This is where MCM differs from ordinary frequency division multiplexing, where the spectra are not permitted to overlap.
MCM provides, among other advantages, greater immunity to noise and interference by distributing the signal among the several carriers which are distributed uniformly throughout the allocated frequency band.
A useful interference characteristic of composite television signals (e.g. NTSC) is that harmonic clusters of the chrominance signal are "frequency interleaved" in the luminance spectrum and simultaneously transmitted in compatible color television systems. Tests have shown that a first signal which is at a harmonic of the line-scanning rate, and a second signal at an odd harmonic of half the line-scanning rate which has approximately five times the voltage of the first signal, are equally visible. Therefore, if a television signal consisting of a low power carrier and a higher power carrier with a 14 dB difference between them, were properly interleaved in frequency with the spectrum of an NTSC co-channel signal, it would cause approximately the same degree of interference into the NTSC signal as would a co-channel signal having two lower power carriers. Thus a correlated co-channel signal can be more robust than a non-correlated lower power signal yet substantially equal to it with respect to interference with the NTSC signal.
The Waters et.al. reference incorporated herein, describes the use of orthogonal frequency division multiplex (OFDM) modulation for digital audio broadcasting (DAB). Waters hints that by carefully choosing the sub-carrier spacing (related to the television line spacing) it might be possible to interleave digital audio signals with geographically separated, co-channel analog television signals.
The instant invention has as one object, to provide a method and apparatus which takes advantage of the benefits provided by MCM techniques in television systems such as those described in the parent applications.